Gas turbine engines typically include a compressor section, a combustor section and at least one turbine that rotates in order to generate electrical power. The compressor discharge feeds directly into the combustor section where hydrocarbon fuel is injected, mixed and burned. The combustion gases are then channeled into and through one or more stages of the turbine which extracts rotational energy from the combustion gases. In order to achieve maximum operating efficiency, gas turbine combustion systems operate over a wide range of different fuel compositions, pressures, temperatures and fuel/air ratio conditions, preferably with the ability to use either liquid or gas fuels or a combination of both (known as “dual fire” systems). However, many candidate hydrocarbon fuels for use in gas turbine combustors contain unwanted contaminants and/or byproducts of other processes that tend to inhibit combustion and/or reduce the capacity and efficiency of the system.
In recent years, the abatement of emissions, particularly NOx, has also gained increased attention in the U.S. due to strict emission limits and environmental pollution control regulations imposed by the federal government. In the burning of a hydrocarbon fuel, the oxides of nitrogen result from high temperature oxidation of the nitrogen in air, as well as from the oxidation of nitrogen compounds, such as pyridine, in the hydrocarbon-based fuels.
Some progress has been made in reducing NOx emissions in gas turbine engines using exhaust gas recirculation due to the “vitiation effect,” which causes the combustor inlet oxygen concentration to be reduced and the CO2 concentration and moisture content to increase compared to a non-recirculation system. Because the rate of formation of NOx is strongly dependent on peak flame temperature, a small decrease in flame temperature tends to lower the NOx emissions. One known technique involves recirculating the exhaust gas to the gas turbine engine which results in additional CO2 being formed, but with only incremental decreases in the O2 and CO concentration. Unfortunately, the amount of oxygen remaining in the exhaust gas using exhaust gas recirculation invariably remains at or above about 2% and it is well known that excess amounts of oxygen can adversely effect the efficiency of most NOx removal catalysts. Thus, previous efforts to reduce and/or eliminate NOx in the exhaust stream using recirculation have met with only limited success.
Another concern in applying exhaust gas recirculation to a stationary gas turbine engine involves the need to reduce the exhaust gas temperature and avoid increasing the combustor inlet temperatures or compressor load when the inlet stream is combined with the recirculation. Commonly owned U.S. Pat. No. 4,313,300 teaches that the problem of excess heat can be substantially overcome if the power plant includes a combined gas turbine steam turbine system with the recycled gases being introduced into the single air compressor supplying air to the gas turbine combustor. However, the '300 patent does not teach or suggest using data regarding the carbon monoxide present in the exhaust gas to adjust the amount of fuel being fed to the combustors, operating either alone or in tandem with other combustors.
Heretofore, monitoring carbon monoxide emissions to control the fuel to air ratios of individual selected combustors in a gas turbine engine has not been used, particularly through the use of feedback control or tuning circuits such as those described herein. Nor do known prior art gas turbine systems provide an acceptable method for fine-tuning the fuel-to-air ratio on an individual combustor-by-combustor basis in order to reduce the amount of CO and oxygen present in the exhaust. Examples of known prior art gas turbine systems include U.S. Pat. No. 6,598,402 to Kataoka et al which discloses an exhaust gas recirculation-type gas turbine that recycles a portion of the exhaust gas to the intake of a compressor and a recirculation control unit for adjusting the amount of gas being returned to correspond to the change in load of the gas turbine. The '402 patent does not rely on the amount of carbon monoxide detected in the exhaust stream as a means for adjusting the fuel to air feed to selected combustion units. Nor does the patent teach how to provide for stoichiometric exhaust gas recirculation control.
U.S. Pat. Nos. 6,202,400 and 5,794,431 to Utamura et al describe two different, but related, methods for improving the thermal efficiency of a gas turbine and steam turbine combination whereby a portion of the gas turbine exhaust is recirculated to the compressor in order to help maintain a more uniform and constant compressor feed temperature and improve the overall thermal efficiency of the system. Neither patent teaches using a tuning fuel circuit or feedback control to effectively reduce the CO or oxygen content of the exhaust based on the detected amount carbon monoxide in the exhaust stream. In addition, the mere detection of CO does not address the need for stoichiometric exhaust gas recirculation control.
PCT application Serial No. WO 99/30079 describes a heat recovery steam generator for use with the exhaust of a gas turbine engine that includes an air pollution control assembly comprising a selective catalytic reduction catalyst for reducing the amount of exhaust gas emissions, including NOx and CO. Again, the '079 application does not teach or suggest using the detected amount of CO in the exhaust as a means for controlling selected fuel inputs to the combustors or teach how to provide for stoichiometric exhaust gas recirculation.